Ratiometric measurement techniques are favored in the measurement of various parameters affecting the transmission of light through a particular milieu. These techniques provide the stability and accuracy necessary to detect minute changes of conditions solely in the milieu divorced from changes in the optical apparatus with time and temperature.
It has been found convenient to subject the reference specimen and the unknown specimen to separate light sources of difference types, but matched intensity.
For instance, the unknown specimen can be exposed to a steady light source while the known specimen is subjected to a flashing light. The light beams transmitted through both specimen are then combined and measured and through a common optical device. The intermittent character of the resulting signal obtained from the flashing light source facilitates its separation from the continuous output signal corresponding to the steady light source, and allows the development of ratiometric information to automatically compensate for spurious apparatus variations such as light intensity, photocell sensitivity, and amplifier gain.
The two light sources are usually generated from a common one which has been split by prismatic refraction. The flashing light is usually obtained by interposing a vibrating mechanical chopper, or perforated rotating disk, in the path of one of the twin light sources. Such mechanical systems are often cumbersome and difficult to maintain in good running order.